Polarization division multiplexed quadrature phase shift keying (PDM-QPSK) is considered an attractive option for 100-Gb/s optically routed wavelength division multiplexed (WDM) transport systems. PDM-QPSK provides a spectral efficiency SE of about 3.2 bits/s/Hz in point-to-point applications using direct detection, and about 2.0 bits/s/Hz in an optically routed environment using coherent detection, where SE is defined as the net per-channel bitrate Rb divided by WDM channel spacing Δf. That is, PDM-QPSK enables an optically-routed networking at 100 Gb/s on a 50-GHz WDM grid (SE of 2.0 bits/s/Hz).
Due to the continuing growth in bandwidth demands, it is desirable to provide optically routed transport systems well above 100 Gb/s with SEs well above 2 bits/s/Hz.